Method name inconsistency with TomCrypt API

[mikeboers]

It has been suggested by a user that the ECC and RSA key.as_string() method name is not similar enough to the TomCrypt function names (*_export) for the connection to be immediately obvious.

I propose adding export() methods where appropriate, deprecating (but not removing) the old names.

[mikeboers]

Another idea is to add __str__() and __iter__()/__getitem__() to replace both as_string() and as_dict(), respectively, in the hopes of making it even more "Pythonic".